Previous work in this laboratory has emphasized the use of frequency-domain measurements for optical imaging. We have now extended this work to include the analog of emission tomography with diffusing waves. In conventional emission tomography the spatial distribution of a radioactive source is imaged using a variant of standard tomographic image reconstruction. Here emission tomography refers to the reconstruction of a fluorescent source distribution which has been excited by a diffusing wave. In previous work we have developed a direct reconstruction algorithm for diffusive emission tomography. Briefly, the number density of a fluorescent source in a highly-scattering medium may be reconstructed by numerically solving an integral equation that relates the quantum efficiency of the exciting diffusing wave to the number density. We have demonstrated the experimental validation of this method in model systems whose optical properties are similar to those of biological tissue. Ultimately, this method could be applied to imaging tumors or other structures tagged by a fluorescent probe.